ONCOLOGY: MAST CELL TUMORS IN DOGS

THUMBNAIL SKETCH
The most common presentation and management at YVC for dogs with mast cell disease is:

We examine a lump in or under the skin, and then test it with cytology. (We collect some cells from the lump and examine them under the microscope. Mast cells are relatively easy to identify.)

We surgically remove the lump. This is typically a day procedure; dogs are hospitalized on the morning of their surgery day, and discharged in the mid to late afternoon of the same day.

We send the removed lump to the diagnostic lab for evaluation as a biopsy. (This step is necessary to 1) confirm that it is a mast cell tumor and 2) determine if it is low, intermediate, or high grade.)

About 80% to 90% of mast cell tumors that we diagnose are low or intermediate grade, and surgery is curative.

A small but significant percentage of the dogs that we see with mast cell disease have a high-risk version of mast cell disease and high grade mast cell tumors. The following is a more extended article about this problem.

BIOLOGY

Mast cells are a normal part of a dog’s immune system.

Normal mast cells can genetically mutate into cancerous cells. When this happens the most common result is a mast cell tumor (MCT), but, rarely, malignant mast cells can infiltrate body tissues microscopically or cause mast cell leukemia (malignant mast cells in the blood).

We do not know what causes normal mast cells to mutate into abnormal ones. It seems most likely that it is due to a combination of an individual dog’s genetic predisposition to having this happen and some environmental trigger(s).

Infrequently mast cell disease can cause general symptoms, including vomiting, diarrhea, blood in the stool, edema of legs, and fever. Very infrequently, a dog can suffer a shock-type collapse.

DIAGNOSIS

Because of the great variation in how it presents to us, we cannot diagnose mast cell disease with a history and examination alone.

Cytology is examination of cells, microorganisms and other material microscopically. We commonly perform cytology on skin tumors and other growths by using a small needle to collect a sample that we smear on a microscope slide, stain, and examine here at YVC. Mast cells are relatively easy to identify. Cytology is a very sensitive and very useful test for diagnosing mast cell tumors.

But cytology is not a perfect, fool-proof test. We have encountered tumors that we examined with cytology and did not find mast cells, only to re-examine them days, weeks, months, or even years later, and found mast cells at this later time. Because of this, it is not uncommon for us to repeat cytology for dogs that have tumors present for a long time.

We believe it is important to evaluate lymph nodes in the region of the tumor with cytology or with a surgical biopsy. This may be easy to do or very difficult. This may be done before, at the time of, or after primary tumor removal.

Depending upon the particular situation other diagnostic tests may be critical for accurately assessing the pet. These tests include a general blood profile, chest x-rays and abdominal ultrasound, and special staining and dna tests of biopsy samples. These tests may be done before, at the time of, or after primary tumor removal.

Biopsy with histopathology (microscopic examination of the removed tumor or section of tissue by a pathologist) is the only way to obtain a detailed diagnosis, and it is essential when follow-up treatment will involve anything more than observing the pet for re-growth of the tumor.

PROGNOSIS

The prognosis is a prediction or forecast of the progress and outcome of medical problem. These are some of the prognostic factors for MCTs:

Lymph node involvement probably indicates a worse prognosis.

The presence of multiple MCTs does not necessarily indicate a worse prognosis.

MCTs under the toenail, in the mouth or throat, on the eyelids, or on the edge of the prepuce, vagina, or anus generally have a worse prognosis; MCTs of the skin generally have a better prognosis. MCTs of internal organs have a worse prognosis.

Symptoms in addition to the simple presence of the tumor itself indicate a worse prognosis.

Boxers, Boston Terriers, and Pugs usually have MCTs with a better prognosis; Shar-peis usually have a worse prognosis.

At YVC it is our impression that MCTs that are relatively large and/or are growing rapidly are more likely to have a worse prognosis.

In a study, 306 dogs that had skin MCTs that were surgically removed had the following survival probabilities:

95% 6 months
93% 1 year
92% 2 years
86% 5 years

Other findings from this study:

9% had MCT-related deaths
4% had MCT metastasis
8% had local re-growth of the removed tumor
12% of incompletely removed tumors grew back

TREATMENT OF LOW-RISK MAST CELL DISEASE

Surgical removal of the tumor and possibly the lymph node(s) in the region of the tumor is the treatment of choice for MCTs of the skin. It is often curative.

We remove as much apparently normal tissue around the tumor as possible, with the goal of removing the tumor entirely. Mast cells sometimes spread microscopically into the tissues surrounding a tumor; we will sometimes remove a tumor with what appears to be wide margins of normal tissue, only to have biopsy results of mast cells at the margins of the removed tissue.

When a tumor is determined to be of low or intermediate grade and is incompletely removed, follow-up options include radiation therapy, chemotherapy, or observation alone.

We recommend monitoring patients that have had MCTs removed by examining them at 3, 6, 9, and 12 months after surgery, and examining and performing cytology on new lumps that appear at any time.

TREATMENT OF HIGH-RISK MAST CELL DISEASE

When any of the factors described in the prognosis section indicate a worse prognosis, when a skin MCT occurs in a location from which it cannot be easily removed, and/or when a biopsy indicates a tumor is high grade, we believe the patient has high-risk mast cell disease.

Surgery is still the treatment of choice for many patients with high risk mast cell disease.

Radiation is an option for MCTs that cannot be surgically removed, or that can only be partly removed. Radiation is not currently available for pets in Maine; the closest specialty practices that offer this mode of treatment are in New Hampshire and Massachusetts. A small number of MCT patients from YVC have had radiation following surgery; this treatment has been very well tolerated and very effective.

Follow-up treatment with chemotherapy is important to consider with high-risk mast cell disease.

Chemotherapy is not very effective when used as the only mode of treatment, but can be very effective when combined with surgery.

There are a few different options for chemotherapy:

Traditional chemotherapy with intravenous plus oral medications, if combined with surgery, has provided average survival times of more than 2 years in most studies. The initial round of chemotherapy costs about $2000.

Tyrosine kinase inhibitors (TKIs) are oral chemotherapy agents that can be used for mast cell tumor control. They are typically given every other day or 3 days per week for an indefinite time period. The positive response rate in one study of 86 dogs with intermediate or high grade mast cell tumors was 37.2 %. TKIs cost about $500 to $600 per month.

Palliative therapy is medical treatment to relieve symptoms, without attempting to treat the cause of the problem. We believe palliative therapy is important alternative to chemotherapy to consider for dogs with high-risk mast cell disease and intermediate to high grade mast cell tumors.